The present invention relates to a fluid machinery, and more particularly to a fluid machinery which includes a centrifugal pump arranged to easily provide constant-flow-rate characteristics suitable for a circulation pump, and an axial-flow pump arranged to easily provide constant-pump-head characteristics suitable for a water supply pump.
Heretofore, centrifugal pumps have been used as cold or hot water circulation pumps in heating or cooling applications. Important factors to be taken into account in this heating or cooling applications are as follows:
{circle around (1)} Even if a required flow rate is known, since there is a slight difference between a calculated pipe-induced loss and an actual pipe-induced loss, the fluid flow rate needs to be adjusted by a valve at site. In this case, the fluid flow suffers an energy loss commensurate with a loss caused by the valve.
{circle around (2)} When the pipe-induced loss increases due to aging of the pipe, or clogging of the valve caused by foreign matter, the flow rate is reduced. Therefore, it is necessary to adjust the flow rate periodically by the valve or the like.
{circle around (3)} Because no means for measuring the flow rate is generally available at site, it is necessary to know the pressure with a pressure gage or the like and estimate the flow rate based on a pump characteristic curve. However, this process is low in accuracy.
Conventional techniques for solving the above problems are set forth as follows:
{circle around (1)} A signal from an electromagnetic flowmeter is processed by a control console, and the opening of a solenoid-operated valve is controlled. Since this process is expensive and accompanied by a loss caused by the valve, its energy-saving effect is small.
{circle around (2)} A signal from an electromagnetic flowmeter is sent to a frequency converter for operating the pump at variable speeds. This process has an energy-saving effect, but is expensive.
{circle around (3)} The pump has a rotational speed selecting knob which is used to change Q-H characteristics of the pump and also to meet a required flow rate in combination with a valve. This process is effective to reduce an energy loss due to the resistance imposed by the valve, but is not effective to stabilize the flow rate. If there is an increase in the pipe-induced loss, then, the flow rate needs to be adjusted each time the pipe-induced loss increases.
In view of the above problems, it is therfore an object of the present invention to provide a fluid machinery such as a centrifugal pump or the like which requires no special auxiliary facilities and supplies a stable flow rate at all times regardless of changes in the resistance imposed by the pipe.
Another object of the present invention is to provide a fluid machinery such as an axial-flow pump which generates a constant pump head even when the flow rate varies, and is suitable for use as a water supply pump.
In order to achieve the above object, according to the present invention, there is provided a fluid machinery for generating a pressure by rotating an impeller with a motor, comprising: a frequency converter for supplying electric power to the motor; a detector for detecting a frequency and a current value; and a program for specifying in advance the relationship between the frequency and the current value; wherein a frequency and a current value in an actual operation are compared with the specified program, and the frequency generated by the frequency converter is varied so that an operating point of the fluid machinery approaches the specified program.
According to an aspect of the present invention, the fluid machinery is such a type that a shaft power increases as a flow rate increases at a constant rotational speed, and the flow rate of the fluid machinery is controlled so as to be substantially constant even when a generated pressure varies.
According to an aspect of the present invention, the fluid machinery is such a type that a shaft power decreases as a flow rate increases at a constant rotational speed, and a generated pressure is controlled so as to be substantially constant even when the flow rate varies.
According to an aspect of the present invention, the frequency (Hz) and the current value (A) are related by a unique function and programmed.
For example, the relationship is represented by A=KHzn (where K and n represent positive constants). The frequency converter has means for changing values of K and n.
According to the present invention, there is also provided a pump assembly comprising: a centrifugal pump driven by a three-phase induction motor; a frequency converter for supplying electric power to the three-phase induction motor; a detector provided in the frequency converter for detecting a frequency and a current value; and a program for specifying the relationship between the frequency and the current value which is stored by the frequency converter; wherein a frequency and a current value in an actual operation are compared with the specified program, and the frequency generated by the frequency converter is varied so that an operating point of the pump is closer to the specified program, and a flow rate is controlled so as to be substantially constant even when a pump head of the pump varies.
According to an aspect of the present invention, the pump assembly has a function for multiplying time outputted from the frequency converter by the value of the constant flow rate for thereby calculating the flow rate. The frequency converter has an indicator for the flow rate.